The transfer capacity required in data transfer is continuously increasing, particularly in cellular radio Systems In the pan-European GSM mobile communications system, for example, there is need, in addition to the presently-used data transfer rates of 9.6 kbit/s and 4.8 kbit/s, for higher transfer rates, such as 14.4 kbit/s, which is required by data services of the public switched telephone network PSTN, for example the modem and telefax terminals of class G3.
FIG. 1 in the accompanying drawing shows a simplified block diagram of the pan-European GSM mobile communications system. The network subsystem (NSS) comprises a mobile services switching centre MSC which communicates with other mobile services switching centres, and either directly or via the system interface of a gateway mobile services switching centre (GMSC), the mobile communications system is connected to other networks, such as the public switched telephone network (PSTN), an integrated services digital network (ISDN), other mobile communications networks such as the public land mobile network (PLMN) and packet-switched public data networks (PSPDN) and circuit-switched public data networks (CSPDN). The mobile services switching centre comprises network interworking functions (IWF) by means of which the GSM network can be adapted to other networks. The IWF comprises an echo cancellation part, modems for modulating the signal originating from the mobile communications network as required before sending it over the system interface to other networks and, correspondingly, for demodulating the signal received from other networks into a PCM signal. In addition, the IWF comprises rate adaptation for adapting the transfer rate to be suitable for other networks, and, correspondingly, for adapting the signal rate from other networks for the GSM network. The network subsystem NSS is connected via the A-interface to the base station subsystem (BSS) which comprises base station controllers BSC, each controlling the base stations BTS that are connected to them. The interface between the BSC and the base stations BTS connected thereto is the Abis interface. The base stations BTS for their part communicate on the radio path with mobile stations MS over the radio interface.
The transcoder/rate adaptor unit (TRAU) is a part of the base station subsystem BSS and may be located at the base station controller BSC, as shown in FIG. 1, or alternatively at the mobile services switching centre MAC. The transcoders convert speech from a digital format to another, for example 64 kbit/s PCM received over the A-interface from the MSC, into data to be transmitted to the base station, and vice versa. One 64 kbit/s PCM channel carries four speech/data connections, which means that the rate of one speech/data channel on this link is 16 kbit/s.
In FIG. 1, between the MS and the data terminal 12, here represented by a PC, there is the TAF block (Terminal Adaptation Function) whose function is to carry out the adaptation between the data terminal 12 and the radio sections of the MS so that the bit stream originating from the terminal equipment is adapted to the radio path. The mobile station MS connected to the data terminal 12 transmits user data over the radio interface on the radio channel at 9.6 kbit/s or 4.8 kbit/s, as specified in the standard. The base station BTS receives the data of the traffic channel and transfers it to the 64 kbit/s timeslot of the PCM circuit. In addition, the three other traffic channels of the same carrier are inserted into the same timeslot, i.e. channel, resulting in that the transfer rate per connection is 16 kbit/s. At the BSC, the TRAU converts the coded 16 kbit/s digital information into the 64 kbit/s channel, and on this channel the data is transferred into the IWF unit at the MSC. The IWF carries out the necessary modulation and rate adaptation, after which the data is transmitted to some other network. Thus, user data is transferred over fixed connections in the uplink direction from the BTS to the BSC and the MSC, and correspondingly, the data to be transmitted to the MS is transferred in the downlink direction from the MSC via the BSC to the BTS and from thereon over the radio interface to the MS. The channel codec unit (CCU) of the base station carries out the conversion of the signal received on the radio channel into the channel of the PCM timeslot in the trunk circuit over the Abis interface, and the conversion of the signal received over the Abis interface into the form transmitted to the radio path. The TRAU carries out the conversion operations for the signals to be transferred over the A-interface.
The user data is transferred over the Abis; interface from the BTS to the TRAU in a fixed-length TRAU frame. The TRAU frame comprises 40 octets numbered 0, . . . ,39, its total length thus being 320 bits and duration 20 ms. FIG. 2 shows in bit diagram form a TRAU data frame used to transfer a signal at the data rate of 9.6 kbit/s or 4.8 kbit/s. Synchronization between the unit that transmits the TRAU frame and the one that receives it is achieved with synchronization bits that are shown in FIG. 2 as 0-bits and 1-bits. The 0-bits in the first two octets of the TRAU data frame are used for carrying out the actual synchronization, and the 1-bits in the first bit position in the other octets except the first, second, and fourth, are used to ensure that elsewhere in the data frame there are no two-octet-long sequences of successive 0-bits that would look like a synchronization sequence. The TRAU frame of FIG. 2 shows control bits C1-C15, and the user data bits denoted with X. Control bit C6 is used to transfer information on the data rate, such as 8 kbit/s or 16 kbit/s. In the figure, the octets containing user data are separated by dotted lines into sections containing 63 data bits, the total length of such a section, including the synchronization bits, being 72 bits. Unused data bits are set to 1-state, for example, for the duration of breaks in the data transmission.
The TRAU frame according to FIG. 2 is as such unsuitable for transfer of signals at 14.4 kbit/s, because 288 data bits would have to be inserted into one frame. For such data transfer, an extended TRAU data frame has been proposed whose structure is shown by FIG. 3. An extended TRAU data frame is also suitable for transfer of signals at 7.2 kbit/s. In FIG. 3, the user data bits D3-D286 are for the sake of simplicity marked with X. The 288 bits in a 14.4 kbit/s data rate signal are thus inserted into user data bits D1-D288. So, the synchronization bits in the first bit position of octets 4-39 in the normal TRAU data frame have been replaced by user data bits in the extended TRAU frame, and the control bits C14 and C15 have been replaced by bits S1 and S2. In non-transparent data, the S1 bit is used to carry information on the number of the half in multiframe transmission, and the S2 bit is used to convey information on discontinuous transmission DTX, and for transparent data the S bits indicate the multiframe numbering. As it has been necessary to reduce the number of synchronization bits in the extended TRAU data frame, a synchronization frame formed from a normal TRAU data frame of FIG. 2 by setting the user data bits to 1-state is used in a separate synchronization procedure, to be described below, in order to ensure synchronization when 7.2 and 14.4 kbit/s signals are transferred. FIG. 4 shows a synchronization frame thus formed. In the synchronization procedure, the channel codec unit CCU of the base station BTS transmits at the beginning of a 7.2 or 14.4 kbit/s transmission a synchronization frame of FIG. 4 to the TRAU, the control bits of the synchronization frame indicating the frame type used, e.g. extended 14.4 kbit/s or extended 7.2 kbit/s. The transcoder TRAU responds with an identical synchronization frame, after which the CCU begins the actual transmission by transmitting an extended TRAU data frame. Data transfer continues to both directions by transmitting extended TRAU data frames. In addition, the synchronization frames are transmitted as described above when the data rate on the traffic channel changes during the transmission from another data rate to the rate of 7.2 or 14.4 kbit/s.
For data transfer to be successful, the transmitting and receiving units must be mutually synchronized. In the transfer of TRAU frames, the transcoder unit TRAU receives its synchronization from the BTS. In case the synchronization is lost in the middle of data transfer, the transcoder TRAU transmits information on it to the BTS in the TRAU frame to be transmitted next by using an Uplink Frame Error (UFE) parameter. The UFE parameter is disclosed to be inserted into control bit C6 in the extended TRAU frame, the control bit C6 being in 1-state during normal synchronization and in 0-state when synchronization in the uplink direction hag been lost. The channel codec unit at the BTS reacts to receiving the UFE parameter by transmitting a synchronization frame to the TRAU, acknowledged by the TRAU by a corresponding synchronization frame. If the CCU detects synchronization loss in the downlink direction, it starts a corresponding synchronization procedure.
User data at 9.6 kbit/s and 4.8 kbit/s in the transmission line between the IWF of the MSC and the TRAU unit in the BSS is normally transferred in an 80-bit V.110 frame according to the ITU-T Recommendation V.110, the structure of such a frame being shown in FIG. 5. The frame comprises 10 octets numbered 0, . . . , 9. At the transfer rate of 16 kbit/s, the frame duration is 5 ms. The bits of the first octet and the first bit of every octet are synchronization bits. The 0-bits in the first octet constitute the actual frame synchronization, and as in a TRAU frame, the 1-state of the first bit in the other octets is used to ensure that elsewhere in the frame there are no eight successive 0-bits that might be taken for frame synchronization. During breaks in the data transmission, an idle V.110 data frame shown in FIG. 6 is transmitted, all the data bits of which have been set to 1-state. The D1 and X bits in FIG. 5 are data bits, 63 of which can be accommodated by a single V.110 frame. Thus, four V.110 frames are required to transfer the bits of a normal TRAU data frame.
The 4*72 bits to be transferred over the Abis interface in the extended TRAU frame in 14.4 kbit/s user data do not fit into the four V.110 frames of FIG. 5. Draft recommendations disclose the TRAU frame to be also transferred in the 64 kbit/s channel over the A-interface. If, however, it is desired that 16 kbit/s V.110 frames are used to transfer data in this 64 kbit/s channel, the problem arises that an extended V.110 frame cannot be formed in a similar way as the extended TRAU frame was assembled by converting the synchronization bit in the first bit position of the frame octets into a data bit, because the remaining few synchronization bits of a V.110 frame would not be adequate to ensure reliable synchronization. In addition, the synchronization frame formed correspondingly in connection with the imaginary extended V.110 frame according to the synchronization frame used in connection with the extended TRAU frame would be identical to the idle V.110 frame shown in FIG. 6, because the V.110 frames do not include any control bits whatsoever that would indicate the data rate of the information. In such a case, the unit that receives the frame could not distinguish the synchronization of the extended V.110 frame from a normal V.110 idle transmission.